In the following, a tissue paper product relates to an absorbent paper based on cellulose wadding which is also called tissue paper base-sheet in this field of technology. A typical absorbent paper has a low basis weight, in the range from 10 to 45 g/m2.
The tissue paper can be produced from paper fibers according to the Conventional Wet Press (CWP) manufacturing method, or by the Through Air Drying (TAD) manufacturing method, or any alternative manufacturing method (e.g. Advanced Tissue Molding System ATMOS of the company Voith, or Energy Efficient Technologically Advanced Drying eTAD of the company Georgia Pacific). The paper fibers can be produced from virgin and/or recycled paper pulp raw material.
The CWP manufacturing method includes the steps of:                pressing and drying the wet paper fibers as a sheet on a large-diameter, heated cylinder (also called Yankee dryer); and        subsequently detaching and creping the sheet of dried paper fibers by means of a metal blade applied against said cylinder, across its direction of rotation.        
The creping operation creates undulations in the sheet across its direction of travel. The creping operation increases the thickness of the sheet, and confers elasticity and gives touch properties to the sheet.
The TAD manufacturing method includes the steps of:                molding the sheet of wet paper fibers on a fabric; and        subsequently drying the sheet, at least partly, by means of a current of hot air passing through it.        
Subsequently, the dried sheet may be creped.
Once, the tissue paper has been manufactured, a distinct manufacturing operation called converting operation is necessary to form the end product (i.e. the paper towel, toilet tissue rolls, bathroom tissue, wiping tissue, kitchen tissue rolls, handkerchiefs, etc . . . ). During the converting operation, several of such sheets, also called plies, can be combined to form said end product.
It is possible to combine several plies together to confer particular properties on a sheet such as thickness, softness, and bulkiness.
Several plies may be combined together by a combining operation of a chemical nature (e.g. by adhesive bonding), or of a mechanical nature (e.g. by knurling or embossing), or a combination of both. During adhesive bonding, a film of adhesive is deposited over some or all of the surface of one of the plies, then the adhesive-treated surface is placed in contact with the surface of at least one other ply. During the mechanical combination, the plies may be combined by knurling, or by compression, or by embossing. Embossing is a deformation in the thickness of the ply or of the multiple plies. It results in a ply having a particular relief or indentation. The thickness of the ply or of the multiple plies is increased after embossing compared with its initial thickness.
The document U.S. Pat. No. 7,497,923 describes multi-ply tissue products having greater tactile sensation and resiliency in hand. The tissues may have a thickened and reduced density middle layer. The tissues may serve as applicators for chemical agents to be released during use of the tissue. A tissue having an improved tactile impression to the consumer, with enhanced resilience and high external bulk is disclosed. In one embodiment, a multi-ply structure having at least three plies is desirable. A middle or intermediate ply has an increased thickness and a greater bulk. In general, the exterior plies of the tissue are smooth and desirable to the consumer.
There is a need to improve the thickness, softness, bulkiness, absorption capacity and strength of the multi-ply tissue products. Further, this should be obtained by using less paper fibers resulting in economical and environmental positive aspects.